Optical-based lithographic techniques are widely employed in the fabrication of integrated circuits (ICs) and other devices requiring very fine-dimensioned patterns or features. However, the constantly increasing demands of micro-miniaturization for increased data storage and computation require fabrication of devices with ever smaller dimensions, which demands tax or even exceed the limits of conventional optical lithographic patterning processes utilizing visible light. As a consequence, intense research has been conducted on ultra-violet (UV), X-ray, electron beam (e-beam), and scanning probe (SP) lithography. However, while each of these techniques is capable of providing high resolution, finely-dimensioned patterns and features, the economics of their use is less favorable, due to such factors as limitations arising from wavelength-dependent phenomena, slow e-beam and SP writing speeds, and difficulties in the development of suitable resist materials.
Thermal imprint lithography has been recently studied and developed as a low cost alternative technique for fine dimension pattern/feature formation in the surface of a substrate or workpiece, as for example, described in U.S. Pat. Nos. 4,731,155; 5,772,905; 5,817,242; 6,117,344; 6,165,911; 6,168,845 B1; 6,190,929 B1; and 6,228,294 B1, the disclosures of which are incorporated herein by reference. A typical thermal imprint lithographic process for forming nano-dimensioned patterns/features in a substrate surface is illustrated with reference to the schematic, cross-sectional views of FIGS. 1(A)-1(D).
Referring to FIG. 1(A), shown therein is a mold 10 (also referred to herein as a “stamper/imprinter”) including a main body 12 having upper and lower opposed surfaces, with a molding (i.e., stamping/imprinting) layer 14 formed on the lower opposed surface. As illustrated, molding layer 14 includes a plurality of features 16 having a desired shape or surface contour. A workpiece comprised of a substrate 18 carrying a thin film layer 20 on an upper surface thereof is positioned below, and in facing relation to the molding layer 14. Thin film layer 20 is typically comprised of a thermoplastic material, e.g., poly(methylmethacrylate), herein abbreviated as “PMMA”, and may be formed on the substrate/workpiece surface by any appropriate technique, e.g., spin coating.
Adverting to FIG. 1(B), shown therein is a compressive molding step, wherein mold 10 is pressed into the thin film layer 20 in the direction shown by arrow 22, so as to form depressed, i.e., compressed, regions 24. In the illustrated embodiment, features 16 of the molding layer 14 are not pressed into the thin film layer 20 for the entire depth thereof, and thus do not contact the surface of the underlying substrate 18. However, the top surface portions 24a of thin film 20 may contact depressed surface portions 16a of molding layer 14. As a consequence, the top surface portions 24a substantially conform to the shape of the depressed surface portions 16a, for example, flat. When contact between the depressed surface portions 16a of molding layer 14 and thin film layer 20 occurs, further movement of the molding layer 14 into the thin film layer 20 stops, due to the sudden increase in contact area, leading to a decrease in compressive pressure when the compressive force is constant.
FIG. 1(C) shows the cross-sectional surface contour of the thin film layer 20 following removal of mold 10. The molded, or imprinted, thin film layer 20 includes a plurality of recesses formed at compressed regions 24 which generally conform to the shape or surface contour of features 16 of the molding layer 14. Referring to FIG. 1(D), in a next step, the surface-molded workpiece is subjected to processing to remove the compressed portions 24 of thin film 20 thereby to selectively expose portions 28 of the underlying substrate 18 separated by raised features 26. Selective removal of the compressed portions 24, as well as subsequent selective removal of part of the thickness of substrate 18 at the exposed portions 28 thereof, may be accomplished by any appropriate process, e.g., reactive ion etching (RIE) or wet chemical etching.
The above-described imprint lithographic processing is capable of providing sub-micron-dimensioned features, as by utilizing a mold 10 provided with patterned features 16 comprising pillars, holes, trenches, etc., by means of e-beam lithography, RIE, or other appropriate patterning method. Typical depths of features 16 range from about 5 to about 500 nm, depending upon the desired lateral dimension. The material of the molding layer 14 is typically selected to be hard relative to the thin film layer 20, the latter typically comprising a thermoplastic material which is softened when heated. Thus, suitable materials for use as the molding layer 14 include metals, dielectrics, semiconductors, ceramics, and composite materials. Suitable materials for use as thin film layer 20 include thermoplastic polymers which can be heated to above their glass temperature, Tg, such that the material exhibits low viscosity and enhanced flow.
Referring now to FIG. 2, schematically illustrated therein, in simplified cross-sectional view, is a typical sequence of processing steps for performing nano-imprint lithography of a metal-based substrate/workpiece, i.e., an Al/NiP substrate/workpiece, utilizing a conventional “master” or stamper/imprinter, e.g., a Ni-based stamper/imprinter. In a preliminary step, a thin film of a thermoplastic polymer, e.g., a poly(methylmethacrylate), i.e., PMMA, is spin-coated on an annular disk-shaped Al/NiP substrate/workpiece, corresponding to substrates conventionally employed in the manufacture of hard disk magnetic recording media. In another preliminary step, a Ni stamper/imprinter having an imprinting surface with a negative image pattern of features, e.g., a servo pattern with lateral dimensions of about 600 nm and heights of about 170 nm, is fabricated by conventional optical lithographic patterning/etching techniques, provided with a thin layer of an anti-sticking or release agent (typically a fluorinated polyether compound such as Zdol™, available from Ausimont, Thorofare, N.J.), and installed in a suitably configured stamping/imprinting tool. In the next step according to the conventional methodology for performing thermal imprint lithography, the substrate/workpiece is placed in the stamping/imprinting tool and heated along with the stamper/imprinter to a temperature above the glass transition temperature (Tg) of the PMMA, i.e., above about 105° C., e.g., up to about 140° C., after which the patterned imprinting surface of the Ni-based stamper/imprinter is pressed into contact with the surface of the heated thermoplastic PMMA layer on the substrate/workpiece at a suitable pressure, e.g., about 10 MPa. As a consequence, the negative image of the desired pattern on the imprinting surface of the Ni-based stamper/imprinter is embossed into the surface of the thermoplastic PMMA layer. The stamper/imprinter is then maintained within the stamping/imprinting tool in contact with the PMMA layer and under pressure for an interval until the system cools down to an appropriate temperature, e.g., about 70° C., after which interval the substrate/workpiece is removed from the stamping/imprinting tool and the stamper/imprinter separated from the substrate/workpiece to leave replicated features of the imprinting surface in the surface of the PMMA layer.
However, two significant problems have been encountered with the use of the above-described thermal imprint lithography technique when utilized with conventionally configured stampers/imprinters for replicating patterns comprised of areas with periodically arranged small features surrounded by areas with no features, e.g., as in the formation of servo sectors of servo-patterned data/information storage and retrieval media, such as hard disk magnetic media.
Referring to FIG. 3, a first problem which is encountered is the random formation of disordered, maze-like features 20′ in non-pattern portions 20NP of the thermoplastic PMMA layer 20 adjacent the regularly-patterned servo sectors 20SERVO, i.e., non-pattern portions 20NP correspond to surface areas of the substrate/workpiece 18 which are intended to be feature-free. It has been determined that formation of such undesired, disordered features 20′ in areas 20NP not intended to be patterned arises from the presence of gaps G between the imprinting surface of the molding layer 14 of the stamper/imprinter 10 and the thermoplastic layer 20 in the non-pattern areas 20NP, presumably due to the presence of long-range attractive forces between the stamper/imprinter 10 and the thermoplastic layer 20.
A second problem is destruction of at least parts of the replicated patterns, e.g., servo patterns, primarily arising from the difference in thermal expansion characteristics of the stamper/imprinter 10 and the substrate/workpiece with the thermoplastic layer thereon (20/18). Previous approaches for circumventing or alleviating this problem have involved the use of stampers/imprinters thermally matched to the workpiece. However, such approach disadvantageously entails costly preparation of suitable stampers/imprinters 10 comprised of materials which are thermally-matched with the substrate/workpiece material(s) 20/18.
In view of the above, there exists a need for improved means and methodology for performing thermal imprint lithography which eliminates, or at least substantially reduces, the above-described problems of undesired, disordered feature formation in unpatterned areas and pattern destruction arising from incompatible thermal expansion characteristics of the stamper/imprinter and workpiece associated with conventional thermal imprint lithography. More specifically, there exists a need for improved means and methodology for accurately and safely imprinting or embossing a pattern, e.g., a servo pattern, in a surface of a resist or other type relatively soft layer on the surface of a substrate for a data/information storage and retrieval medium, e.g., a hard disk magnetic recording medium.
The present invention addresses and solves the above-described problems and difficulties associated with conventional techniques and methodologies for performing thermal imprint lithography for pattern definition in substrate/workpiece surfaces, such as in the fabrication of hard disk substrates with integrally formed servo patterns, while maintaining full compatibility with all aspects of conventional automated manufacturing technology for pattern formation by imprint lithography. Further, the means and methodology afforded by the present invention enjoy diverse utility in imprint lithographic patterning of a variety of substrates/workpieces.